Field of the Invention
The present invention relates to a safety device used in an apparatus, such as a copying machine, provided with an electric heating device, and particularly relates to a safety device which can rapidly cope with an occurrence of abnormality in an electric heating device.
Description of the Prior Art
In copying machines and some kinds of printers, such as a laser printer and the like, an electrostatic latent image is formed on a photosensitive drum, the latent image is developed with toner, and the toner image is transferred onto paper. Thereafter the transferred toner image is fixed. Ordinarily, the fixing of the toner image is carried out by using heat energy. There are various methods of fixing the toner image. One common method used is a fixing device employing a heat roll because of (i) its low deterioration of picture quality and (ii) its high safeness.
FIG. 6 shows an example of the circuit arrangement of a conventionally used fixing device. In the circuit, a solid state relay (SSR) 2, a heater 3, and a thermostat 4 are connected in series to a 100 V commercial power source 1A. Here, the heater 3 is used to heat a heat roll 5 for fixing a toner image. Unlike a mechanical relay, the solid state relay 2 performs a relay function with its solid-state characteristic.
A thermistor module 6 provided with a thermistor as a temperature detecting element lightly touches a surface of the heat roll 5. Surface temperature information 8 of the heat roll 5, produced by the thermistor module 6, is fed to a temperature control portion 9. The temperature control portion 9 consists of a data input buffer amplifier 9A, a data processing portion 9B for processing a signal produced by the data input buffer amplifier 9A, and a data output buffer amplifier 9C for outputting the signal processed by the data processing portion 9B.
The data processing portion 9B is constituted by an A/C converter for analog-to-digital converting of the analog surface temperature information 8, a CPU (central processing unit), a clock generating circuit, and so on. The data processing portion 9B produces a temperature control signal in response to the surface temperature information 8. A temperature control signal 11 produced from the data output buffer amplifier 9C is applied to a control terminal of the solid state relay 2. The current conduction of the heater 3 is controlled by the solid state relay 2.
The thermostat 4 is arranged to open its contact upon detection of a state of overheat of the heat roll 5. In case of such an abnormal state, a loop passing through the commercial power source 1A is opened so as to prevent a current to flow in the heater 3. That is, when a fault has occurring in the temperature control portion 9 allows a current to continuously flow in the heater 3, the thermostat 4 is caused to operate to secure safety of the apparatus.
However, because the fixing device is provided with a heat source, there is a possibility that the temperature in an apparatus such as a copying machine or the like will be raised to a value unsuitable to a photosensitive drum and other parts. Particularly, since a photo semiconductor is used as a photosensitive drum in certain kinds of printers, a photosensitive characteristic may change considerably so as to make it impossible to form a good picture when the temperature of the machine falls out of an allowable temperature range. Therefore, a heat exhausting device (fan) 13 is provided in the machine. A heat exhausting device controlling circuit 14 controls the current fed from a 100 V commercial power source 1B to the heat exhausting device 13, so as to make the heat exhausting device 13 to remove heat, as required, to keep the temperature inside the machine at a value within a substantially fixed temperature range.
As described above, a safety device using the thermostat 4 is used in the fixing device. If a short circuit occurs in an element, such as the solid state relay 2 for feeding power to the heater 3, causing power to be continuously fed to the heater 3, then the thermostat 4 is actuated to operate to stop feeding the power to the heater 3.
When the control for feeding power to the heat source becomes impossible to be correctly carried out in various of situations as described above, measures have been taken to ensure the safety of the apparatus under the thought of "fail safe".
In the conventional safety device shown in FIG. 6, the operational characteristics of the thermostat 4 are much influenced by the flow of air. Thus, the detection of an abnormal situation is considerably delayed when the heat exhausting device 13 is operating. If the detection of the abnormal situation is delayed, a risk arises that deformation in the surface of the heat roll acting as a thermal fixer, or deformation in a peeling pawl always touching the surface of the heat roll so as to peel paper, will occur.
An object of the present invention is to provide a safety device which can rapidly take safety measures to cope with abnormality in an apparatus provided with a thermal fixer as a heat source and a heat exhausting device for externally exhausting heat.